As shown in FIG. 3, a glass bulb 1 for cathode ray tube generally comprises a glass panel 10 serving as a front surface portion, a funnel 20 serving as a back structural member and a neck 30 for mounting an electron gun therein. The glass panel 10 includes a face portion 11 of approximately rectangular shape having an effective screen for displaying an image, and a skirt portion 13 continued from the periphery of the face portion 11 via a blend R portion 12 and having a seal edge surface portion 14 for sealing with the funnel 20.
The funnel 20 includes a top portion 22 extending nearly vertically in the tube axis direction from a seal edge surface portion 21 for sealing with the glass panel, a yoke portion 24 outside of which a deflection yoke is to be attached, and an intermediate body portion 23 connecting the top portion and the yoke portion. A reference line 25 existing in the yoke portion is a hypothetical line used for indicating a reference position of the funnel. In the case of a color cathode ray tube, the glass panel 10 is sealed between the seal edge surface portion 14 of the skirt portion 13 and the seal edge surface portion 21 of the funnel 20 via frit glass and the like. In this context, θ designates a deflection angle in the yoke portion of the funnel 20, and is defined as an angle of divergence of an effective screen diameter D along the diagonal axis of the rectangular face portion 11 viewed from a hypothetical reference point on the reference line 25.
Since the above glass bulb 1 is used as a vacuum vessel of which interior is evacuated to vacuum, a stress caused by a pressure difference between inside and outside of the bulb will act on the outer surface of the glass bulb 1. In the case of the glass bulb which is not a spherical shell, however, as shown in FIG. 2, there arises complicated distribution of stress such that an area of tensile stress denoted by arrows toward outside of the bulb and an area of compressive stress denoted by arrows toward inside of the bulb are present at the same time.
The vacuum tensile stress generated in the glass bulb 1 usually becomes maximum in end regions on the short axis of the glass panel 10, and when a mechanical or thermal shock exceeding a certain degree is applied to the glass bulb 1 from the outside, the glass bulb 1 breaks from the vicinity of the region where the maximum vacuum tensile stress is generated, that is from the region ranging from the end of the face portion 11 to the skirt portion 13 as its origin of breakage, resulting in implosion. Therefore, the glass bulb 1 used in a cathode ray tube is usually designed to have an enough mechanical strength to suppress the vacuum tensile stress to a predetermined value or less.
The distribution of vacuum tensile stress depends on the size and shape of the glass bulb, and thus the shape, wall thickness and the like are usually designed so as to suppress the vacuum tensile stress generated at the seal edge portion between the glass panel and the funnel to less than about 8.4 MPa, which is one standard of mechanical strength required for a glass bulb determined in consideration of safety factors such as shocks applied from the outside.
For this reason, in conventional glass bulbs for cathode ray tube, in order to suppress the vacuum tensile stress to less than the predetermined value while maintaining the mechanical strength, such measures have been taken as increasing the wall thickness of the panel, elongating the skirt portion for relieving and distributing the vacuum tensile stress generated in the vicinity of the skirt portion to thereby reduce the peak value thereof, and the like.
In conventional glass bulbs for cathode ray tube, however, since the weight of the glass is increased because of the increased wall thickness of the panel or the elongated skirt potion, there is a problem that the glass bulb is inferior in operability and workability.
Moreover, the glass panel is formed by press molding of a molten glass gob and then gradually cooled to manufacture the glass panel of room temperature. However, during the continuous cooling process, delay condition occurs in cooling degree of each portion because of the three-dimensional box-shaped structure and uneven distribution of the wall thickness, so that the glass panel is usually cooled with the temperature distribution being uneven, causing the portion whose temperature has dropped below the temperature of the strain point to sequentially solidify. The strain point is the point that viscous flow of the glass is substantially stopped.
As a result, when the glass panel is brought to the room temperature, the distortion appears as warpage of the glass panel which will generally cause inward inclination in the vicinity of the seal edge surface portion on the short axis and long axis of the glass panel and outward inclination in the vicinity of the seal edge portion on the diagonal axis, and tensile stresses remain outside of the seal edge surface portion on the diagonal axis.
Furthermore, also in the frit seal heating process at the time of sealing the glass panel with the funnel, uneven temperature distribution occurs within the glass panel and a temporary distortion which will cause the seal edge surface portion on the diagonal axis to incline outwardly occurs, resulting that a tensile stress is exerted and the glass tends to break due to the resultant with the remaining tensile stress at the time of the molding as described above. This tendency is significantly problematic in a glass panel in which the skirt portion is elongated in a large panel having a flat face portion.
The applicant of the invention has applied an invention relating a glass panel for cathode ray tube in which weight reduction is enabled by shortening the skirt portion while maintaining the mechanical strength as a glass bulb for international patent (PCT/JP99/07135).
FIG. 6 shows a section in the short axis direction of the glass panel for cathode ray tube which comprises the face portion 11 of substantially rectangular shape, and the skirt portion 13 connected with the periphery of the face portion via the blend R portion 12 and having the seal edge surface 14 for sealing with the funnel. In the above international patent application, the glass panel for cathode ray tube having the shape as shown in FIG. 6 is so configured that an effective screen diameter D(mm) of the glass panel along the diagonal axis thereof is in the range of 500≦D<650; an average radius of curvature of the outer surface of the face portion 11 is more than or equal to 10,000 mm in any radial direction passing the center of the face portion; and a distance h (mm) along the tube axis from a contact between the effective screen end of the inner surface of the glass panel and the blend R portion 12 to the seal edge surface 14 at least along the short axis of the glass panel, and a glass wall thickness t (mm) of the seal edge surface 14 satisfy the relationships of: 0.07D≦h≦0.11D, 0.015D≦t≦0.025D and (D/25.4)2≦t×h≦(D/25.4+3)2, or that the effective screen diameter D(mm) of the glass panel along the diagonal axis thereof is more than or equal to 650; the average radius of curvature of the outer surface of the face portion is more than or equal to 10,000 mm in any radial direction passing the center of the face portion; and the distance h (mm) along the tube axis from the contact between the effective screen end of the inner surface of the glass panel and the blend R portion 12 to the seal edge surface 14 at least along the short axis of the glass panel, and the glass wall thickness t (mm) of the seal edge surface 14 satisfy the relationships of: 0.08D≦h≦0.11D, 0.015D≦t≦0.020D and (D/25.4)2≦t×h≦(D/25.4+2.5)2. In such a way, the above international patent application suggests the technique for reducing the weight of the glass panel compared to the prior art by shortening the skirt portion 13, while maintaining the mechanical strength as a glass bulb.
It is an object of the invention to provide a glass bulb for cathode ray tube in which, by combining the condition of the glass panel for cathode ray tube suggested by the above invention of international patent application into a glass bulb for cathode ray tube having a glass panel which is large in size and having a flat face portion, proper relationship between the glass panel and the funnel in length along the tube axis direction is defined, reduction in weight is achieved by shortening the panel skirt portion while maintaining a certain mechanical strength as a glass bulb, a tensile stress which will occur outside the seal edge surface portion on the diagonal axis is reduced, and breakage during frit seal heating process and the like is suppressed.